8 “Every one that does sin, does also iniquity.” Let no man say, Sin is one thing, iniquity another: let no man say, I am a sinful man, but not a doer of iniquity. For, “Every one that does sin, does also iniquity. Sin is iniquity.” Well then, what are we to do concerning sins and iniquities? Hear what He says: “And ye know that He was manifested to take away sin; and sin in Him is not.” He, in Whom sin is not, the same has come to take away sin. For were there sin in Him, it must be taken away from Him, not He take it away Himself.
“Whosoever abides in Him, sins not.” In so far as he abides in Him, in so far sins not. “Whosoever sins has not seen Him, neither known Him.” A great question this: “Whosoever sins has not seen Him, neither known Him.” No marvel. We have not seen Him, but are to see; have not known Him, but are to know: we believe in One we have not known. Or haply, by faith we have known, and by actual beholding have not yet known? But then in faith we have both seen and known. For if faith does not yet see, why are we said to have been enlightened?
There is an enlightening by faith, and an enlightening by sight. At present, while we are on pilgrimage, “we walk by faith, not by sight,” or, actually beholding. Therefore also our righteousness is “by faith, not by sight.” Our righteousness shall be perfect, when we shall see by actual beholding. Only, in the meanwhile, let us not leave that righteousness which is of faith, since “the just does live by faith,” as says the apostle. “Whosoever abides in Him, sins not.” For, “whosoever sins, has not seen Him, neither known Him.” That man who sins, believes not: but if a man believes, so far as pertains to his faith, he sins not.
Source: Homilies on the First Epistle of John (New Advent)